(a) Field of the Invention
The present invention relates to a zoom lens system comprising three lens groups.
(b) Description of the Prior Art
There have generally been known zoom lens systems which comprise two, three and four lens groups. Out of these zoom lens systems, those comprising two and three lens groups are designed relatively compact but have drawbacks that they cannot provide sufficient performance, for example, so high zooming ratios. The zoom lens system comprising four lens groups have, in contrast, excellent performance, i.e., relatively high zooming ratios, and favorably corrected aberrations, are disadvantageous in that they cannot be designed compact.
Among the zoom lens systems comprising four lens groups, the afocal type zoom lens system is known most widely to those skilled in the art. In order to design this type of zoom lens system compact, it has hitherto practised to design its relay lens group compact, for example, by reducing number of the relay lens components. Patents have been applied for zoom lens systems comprising compactly designed relay lens system. These zoom lens systems have favorable performance as the merit of the relatively compact afocal type of zoom lens system, but are still insufficient in their compactness.
FIG. 1 shows the basic composition of the conventional afocal type of zoom lens system comprising four lens groups. As is seen from this drawing, the afocal type of zoom lens system comprising four lens groups is composed of a first lens group I designed as the focusing lens group which is kept fixed even at the time of zooming, a second lens group II designed as the veriator which has negative refractive power and is moved along the optical axis at the time of zooming, a third lens group III designed as so-called compensator which is moved along the optical axis to compensate deviation of the image position caused by moving the second lens group II, and a fourth lens group IV designed as the relay lens group having positive refractive power. That is to say, this type of zoom lens system is so designed as to perform zooming by moving said second and third lens groups.